U.S. Pat. No. 4,142,132, issued Feb. 27, 1979 entitled "Method and Means for Dynamic Correction of Electrostatic Deflector for Electron Beam Tube"--Kenneth J. Harte, inventor and assigned to the Control Data Corporation describes a new method and means for dynamic correction and minimization of aberrations produced in the electron beam of electron beam tubes employing electrostatic deflection systems. In U.S. Pat. No. 4,142,132, a preferred construction of a compound, fly's eye type electron beam tube employing an eight-fold coarse deflector, an objective lens of the fly's eye type employing a number of micro lenslets and a micro deflector, is described together with the method and means for providing correction electric potentials to the eight-fold deflector member for minimizing aberrations of the electron beam spot at the target plane. In addition, U.S. Pat. No. 4,142,132 describes a method and means for deriving a dynamically corrected focus potential from the fine X and Y deflection voltages and applying the dynamically corrected focus potential to the objective lens of the compound electron beam tube. In a copending United States patent application Ser. No. 093,008, filed Nov. 9, 1979, entitled "Beam Tube Having Electrostatic Eight-Fold Double Deflection System with Coarse Deflection Dynamic Focus and Diverging Beam"--Kenneth J. Harte and Edward C. Dougherty, an improved dynamic focus correction system is described which employs components of both the coarse and fine deflection potentials in deriving the dynamically corrected focus potential to be applied to the objective lens of the compound electron beam tube. The disclosures of both U.S. Pat. No. 4,142,132 and copending application Ser. No. 093,008 are incorporated into the disclosure of this application in their entirety.
The EBAM tube systems described and claimed in each of the above noted prior art disclosure employ a classical Einzel lens as the objective lens assembly, as will be discussed more fully hereinafter in connection with FIG. 1 of the drawings. To achieve focus of an electron beam using a classical Einzel lens assembly it is necessary to apply a high voltage energizing potential (typically 4 kilovolts) to the center element of the standard three elements lens as shown in FIG. 1. However, in order to maintain focus of a deflected electron beam on a stationary target plane, it is further necessary to apply a dynamic correction to the focus voltage so that the focal plane of the lens will fall as closely as possible onto the target plane. For applications requiring rapid changes in order to supply such dynamic correction as illustrated in FIG. 1, it is very difficult and expensive to implement circuitry which will rapidly change and very precisely regulate a high voltage power supply. With the arrangement shown in FIG. 1, it is necessary that the high voltage power supply provide a high voltage energizing potential of typically four kilovolts (4 kv) and it is necessary to superimpose upon this high energizing potential a dynamic fine focus correction potential of say .+-.30 to 100 volts and to provide the dynamically corrected focus potential through an amplifier having a typical band width of 1 megahertz. The provision of such circuitry is at best difficult to both build and maintain in operation as well as expensive.
The electronic drive problem discussed in the preceding paragraph has been overcome in the present invention by applying the dynamic component of the focus voltage to a separate entrance lens element rather than to a common center element as will be described more fully hereinafter with respect to the remaining Figures of the drawings.